Time-of-flight mass spectrometer with step-function-controlled field

ABSTRACT

The performance of a time-of-flight mass spectrometer is improved by applying a step voltage to the deflection plates to produce a step-function-controlled electrical field perpendicular to the ion beam. The step-function-controlled field provides ion bunching wherein the number of ions in each packet is independent of mass and the resolution is independent of mass resulting in a sensitivity that is independent of mass.

United States Patent References Cited UNITED STATES PATENTS 9/1952 Stephens Johannes M. B. Bakker Slttingbourne, England [72] Inventor [21] AppLNo. [22] Filed 2,612,607 250/419 TF 3,307,033 2/1967 Vestal 250/4119 TF Apr.13,l970 [45] Patented Jan.l1,l972

FOREIGN PATENTS 8/1957 GreatBritain..........

250/4l.9 TF

[ 73] Assignee Shell Oil Company New York, N.Y. 780,999 {35; Pflomy Primary Examiner-Anthqny L. Birch 20,005/69 Attorneys-T. E. Bleber and J. H. McCarthy 54] TIME-OF-FLIGHT MASS SPECTROMETER W STEP-FUNCT10N-CONTROLLED FIELD 13 Claims, 15 Drawing Figs.

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PATENTED JAN] 1 I972 SHEET 1 0F 4 PATENIED JAN: 1 m2 SHEET 2 [IF 4 INVENTOR:

J- M. B. BAKKER PATENTEBJANI 1 1972 3,634,683

SHEET 3 OF 4 +E FIG. 20 o O FIG. 2b -E 13 'i 1 FIG. 2c y 0 r 'r FIG. 2d 3 o FIG. 2e E FIG. 4

FIG. 3

INVENTOR:

J.M.B. BAKKER Pmmumuwzz $634683 SHEET 0F 4 H In) 'I' "r w mf l FIG. 7 FIG. IO

INVENTOYR:

J-M-B. BAKKER TIME-OF-FLIGHT MASS SPECTROMETER WITH STEP- FUNCTION-CONTROLLED FIELD This invention relates to time-of-flight mass spectrometers.

Time-of-flight mass spectrometers have the following basic components.

1. means for ionizing the substance under investigation 2. means for bunching these ions and focusing the ion beam 3. means for accelerating the ions 4. a drift tube down which the bunches of ions pass and where the lighter and faster-moving ions become separated from the heavier and slower-moving ions 5. a collector, generally incorporating an electron multiplier, which produces peaks of current corresponding to the various types of ion in the sample 6. a cathode-ray tube or other device wherein these peaks of current are displayed visually, or recorded.

According to the present invention a time-of-flight mass spectrometer is provided with means whereby a step-functioncontrolled electrical field perpendicular to the ion beam is employed to provide bunching. This may be achieved by applying to a pair-of deflection plates between which the ion beam passes, a voltage varying in accordance with a step-function. These deflection plates may conveniently be a single pair of deflection plates such as constitute the X- or Y-plates of a conventional cathode-ray tube. In a preferred form of spectrometer in accordance with the present invention, two sets of deflection plates are provided which may conveniently be arranged as the X and Y-plates of a conventional cathode-ray tube. It is however not essential for the electrical fields between the two sets of plates to be at right angles to each other when viewed in a direction parallel to that of the ion beam. Any angular relationship between the fields from to 90 may be used.

The time taken for all the types of ions of a given bunch to actuate the collector is known as the spectrum, and it is necessary for the electrical field to be maintained at a substantially constant value during this spectrum.

In relation to a single electrical field, the ratio of transit time of the ions through this field to the rise time of the step-function is such that the rise time must be approximately equal to or less than half the transit time of the heaviest ions through the field if adequate separation of ion bunches is to be effected.

The spectrometer in accordance with the present invention produces a continuous beam of ions which has a periodic effect on the collector as a result of the change in the perpendicular electric field or fields produced by the controlling step-function and a resulting change in direction of the ion beam.

Another aspect of the present invention comprises a method for repeatedly obtaining a mass spectrum of ions according to the principle of accelerating a group of ions to a certain voltage so as to impart to the ions velocities corresponding to their mass/charge ratio, and measuring the current of ions arriving at a target as a function of the time it takes the various ions of the group to reach the target. A group of accelerated ions is formed time and again by intermittently directing a beam of accelerated ions towards the target by means of an oscillating transverse electrical field, with the aid of a set of deflection electrodes between which an oscillating deflection voltage is applied. The electrical field oscillates between two levels chosen such as to cause the beam to be deflected repeatedly towards, across and beyond the target, spectra as represented by the varying ion current that reaches the target being measured or registered only for groups of ions directed towards the target as a result of (field) changes in one direction of the deflection voltage.

The step-function-controlled voltage must remain substantially constant at its higher or altered value for the duration of the spectrum and means have to be provided to ensure that the deflection of the ion beam when this voltage falls or returns to its original value, does not produce a display on the oscilloscope which interferes with the display arising from the original increase vor change in the step-function-controlled voltage. These means could comprise an additional pulse generator triggered by the fall or return of the step-controlled voltage, together with synchronizing or delay networks which would cause the two traces to be superimposed. This however involves complications in design which are not warranted by the results achieved. It is preferable to arrange the system so that the ion beam deflection occasioned by the fall or return of the voltage to its original value, is without effect on the oscilloscope. This may be done by returning the voltage to its original value only after the spectrum has taken place or by ensuring that the beam deflection arising from the voltage return to its original value does not affect the collector. This last-mentioned mode of operation can conveniently be carried out by arranging for the application to a second set of plates of the step-function-controlled voltage which by its return to its original value, will prevent the ion beam from reaching the collector.

Thus, when the step-function-controlled voltage has the form of a square pulse of a duration greater than the time of flight of ions between the two sets of plates, the leading edge of the square pulse acts to overcome a deflecting bias voltage on both sets of plates to allow ions to travel substantially axially down the apparatus to the collector, the trailing edge of the pulse reestablishes the deflecting bias on the second set of plates and thus prevents ions which have already left the first set of plates from reaching the collector.

One of the principal advantages derived from the mass spectrometer of the present invention is an increase in sensitivity due to the continuous ionization process, a considerable increase in resolution due to the lack of space defocusing and the improved ion bunching, combined with the speed of operation normally associated with a time-of-flight mass spectrometer. Alternatively, the mass spectrometer according to the present invention may be arranged to achieve currently acceptable sensitivity and resolution with a shorter path-offlight than has heretofor been achieved; this permits a more compact instrument to be constructed. The spectrometer of the present invention also offers the possibility of employing a collector slit of variable width by means of which resolution can be improved at the cost of sensibility and vice versa.

The invention is further described with reference to the accompanying drawings wherein FIG. 1 shows a diagram of the entire mass spectrometer in accordance with the present invention;

FIG. 2 shows diagrammatically the effect on the ion beam of the step-function-controlled voltage;

FIGS. 2a, 2b, 2c, 2d, and 2e are curves showing voltages at different times;

FIGS. 3 to 10 show typically oscilloscope displays produced with the mass spectrometer of the present invention.

Turning now to FIG. 1, the spectrometer comprises an ion gun indicated generally by reference 8, an ionization chamber indicated by the reference 1, and a cathode-ray tube structure generally indicated by the reference 2 adrift tube 3, a collector slit 4, a collector indicated by the general reference 5, and an oscilloscope 6, a pulse generator 7 producing a stepfunction-controlled voltage, and supplying a trigger pulse to the oscilloscope. The power supplies for the ionization chamber 1 and cathode-ray tube structure 2 are provided by a power pack indicated generally by the reference 9. High-voltage power supplies l0 and 11 are provided respectively for the drift tube 3 and collector 5. The ionization chamber 1 may conveniently be of the type known as the Nier ionization chamber, and comprises an inlet 13 for the gas or vapor under examination (generally from a gas/liquid chromatographic analyzer) which flows in the direction indicated by the arrows and is passed through an ionization space 14 where it is bombarded with a stream of electrons 15 passing between a filament 17 and an electron trap 18 operating in combination with an ion repeller 19. A collimating magnet, not shown, may conveniently be used to collimate the electron beam and thereby increase the ion production.

A beam of ions derived from the molecules of the original sample leaves the ionization space 14 through the orifice 20 and thereafter passed through the accelerator electrode a.1, the focusing electrode a.2 and the astigmatism electrode :13 of the cathode-ray tube structure. Thereafter it passes a pair of Y-plates 21, a geometry control electrode :1 and a pair of X- plates 22.

After passing down the drift tube 3 the ion beam passes through the collector slit 4 and strikes the cathode 23 of the collector 5 releasing electrons which activate an electron multiplier of known type comprising a field strip 24 and a dynode strip 25 before reaching an anode 26 thereby supplying a current which passes through the resistance 27 to give a voltage impulse to the oscilloscope 6. Voltages and resistance values are indicated at various points in FIG. 1.

The Y-plates 21 are normally biased in such a way that the ion beam does not strike the cathode 23 of the collector 5. The generator 7 produces the step-function-controlled voltage, which may conveniently be a square pulse of 40-volt amplitude and 50 1. duration. If this is applied to the Y-plates 21 only, and the deflecting bias on these plates is overcome, the ion beam swings across the drift tube and ions pass through the slit 4 on to the cathode 23; or a square pulse of 40-volt amplitude and 1 1 duration may be applied to both the Y-plates 21 and the X-plates 22 to achieve the same result. Since these ions have been subject to the same accelerating voltages applied to the ion gun 8 and to the drift tube 3, and each have the same charge, they accelerate in proportion to their mass so that lighter ions reach the collector sooner than the heavier ions Then, assuming that the oscilloscope trace swings from left to right so viewed in FIG. 1, peaks indicating lighter ions will be shown on the left-hand side of the trace and peaks indicating heavier ions will be shown on the right-hand side of the trace.

Referring now to FIG. 2, this shows diagrammatically the ef fect of a step-function-controlled electrical field on the ion beam. A homogeneous monoenergetic ion beam of width B and consisting ofions with mass m and energy eU travels from left to right through an electrical field of strength E produced between a set of deflection plates of effective length l and effective separation D. After which, the ions travel through a space L, assumed to be free of any fields, until the ions reach a slit 4.

Any ions that have passed through the electrical field before the electrical field change has taken place will have experienced, while travelling through the electrical field, a downwardly directing force. The position of these ions is indicated at three different instants in time by n 11,, a a,,, a and a,.

Any ions travelling through the electrical field after an electrical field change has taken place experience an upwardly directing force while travelling through the deflection plates. The position of these ions is indicated at three different instants in time by e,,, e,, e e,, e, and 2' Any ions travelling through the electrical field at the instant an electrical field change takes place will experience both downwardly and upwardly directing forces. The motion of these ions is dependent on their exact position within the electrical field at the time of field change.

The position of eight ions at time T,, is indicated by a 11' c c d ta and 2' If at T an electrical field change takes place then the a ions will have travelled through an electrical field, as shown in FIG. 2a. For the a and a -ions the changeover took place at a time t =t, where t, is the transit time of the ions through the electrical field. The a-ions are therefore not affected by the changeover and these ions will move along the path indicated by a a,, a a',,, a, and a' The 0,, and c -ions are exactly halfway through the electrical field at time T These ions will have travelled through an electrical field, as shown in FIG. 2c. Because, for these ions, the changeover occurred at 1 =%r,, they will experience a downwardly directing force over a period let and an upwardly directing force also over a period M As a result of this there will be no net change in the velocity of the c and c',,- ions, and they will continue to move parallel to their initial direction of travel. However, upon leaving the electrical field the q, and c -ions will have been displaced to a slightly lower position, due to a difference in initial conditions, i.e., when these ions first experienced an electrical field +E they did not have any up or down velocity components, when these ions first experienced an electrical field -E, they had acquired a downward velocity component due to H5. The position of the c-ions at four different instants in time is shown by c ,c ,c ,c c,,, c", c, and c;,.

The e and e -ions are at the entrance of the deflection plates at time T These ions will travel through an electrical field as indicated by FIG. 2e. For these ions the changeover occurred at 1 0, and they will, therefore, experience only an upwardly directing force. The position of the e-ions at three instants in time is given by e e,, e 2' e, and 0' lfa slit 4 is placed in a position as shown in FIG. 2, then of the a, c, and e-ions only the c-ions will pass through this slit. The limiting case ofions which will pass through the slit 4 is indicated by the ions b and d. The b -ion will have travelled through an electrical field as indicated by FIG. 2b. For this ion, the changeover occurred slightly after hi, or 2 k1,. The ion will, therefore, have experienced a downwardly directing force over a time interval slightly greater than /2t,. It will also have experienced an upwardly directed forece over a time interval slightly less than V21 The net result is that ion b will leave the electrical field with a small velocity component in a downward direction. The position of the b-ion at four different instants in time is shown by b b b and h The changeover time 1 is directly related to the position of an ion at the time of the electrical field change. Ion 1),, presents a limiting case in that any ion for which I; is larger than the 1 of ion 12 will have received too much downward acceleration and therefore, these ions will arrive below the slit 4.

The d -ion will have experienced an electrical field while travelling through the deflection plates as indicated in FIG. 2d. For this ion the changeover occurred slightly before /2!,, or t /2t,. As a result, the ion experiences a downwardly directing force over a time interval slightly less than /21,, followed by an upward directed force over a time interval slightly greater than /fil The net result is that ion d will leave the deflection plates with a small velocity component in an upward direction. The position of the d-ion at four different instants in time is indicated by d d d and d Any ions for which t is less than the t of ion d will arrive above the slit 4.

To state the foregoing in other words, a homogeneous beam of ions passes through an electrical field which deflects the beam downwards. A change in the electrical field will produce a deformation of kink in the beam. This kink comprises the ions having positions between the e-ions and the a-ions at the instant of field change. The beam as a whole, including the kink will continue to travel from left to right with the same velocity. Simultaneously, the kink will expand both in the upward and downward direction. The ions at the upper end of the kink (e-ions) will have the largest upward velocity. The ions at the bottom of the kink (a-ions) will have the largest downward velocity. The ions at the center of the kink will have very little, or no, up and down velocity components. Of the ions affected by the field change, only the ions lying within the boundary d c b -c' will pass through the slit 4. Because the left to right velocity is not affected by the step-functioncontrolled electrical field, the position of the ions along this direction does not change, i.e. the projected distance ea=l and bd=l remains constant with time. Some of the ions do, of course, change position in the up-down direction. Furthermore, when the ions reach slit 4 the time duration of the ion bunch emerging from the slit aperture can be directly related to the changeover time 1 In other words, ion b passes first through the slit, ion (i passes last. Therefore, the ion bunch duration is given by Ar=t (b)t (a). Rotation of the slit 4 about an axis perpendicular to the plane of the paper in FIG. 2, so as to make the slit parallel to the direction L' (i.e.

about 5 anticlockwise rotation) will reduce the ion bunch duration in theory by a factor two. Increasing L or decreasing S will reduce At proportionately. So far only a homogeneous ion beam has been considered. When the beam contains ions of different mass, the ion beam motion becomes more complex. 5 Because the electrical forces are only charge dependent and not mass dependent, the spatial relationship of the ions, whatever their mass, will not change. In other words, the limiting case of ions, passing through the slit aperture will still be within the boundary of the c-b-c-d ions. However, the velocity of individual ions will vary as /Um while their position in time varies Thus, whereas FIG. 2 previously showed the position of a homogeneous beam at four different instants in time, we can now regard this figure as showing the position of three different ion beams, all with the same energy, but with different masses, at one instant in time.

In this case the ion bunch b -c d -c' represents a packet of very light ions, the ion bunch b c d c' represents a packet of medium weight ions, and the ion bunch b c,-a ,c represents a packet of very heavy ions. Notice that the time it takes for the heavier ions to travel from left to right has considerably increased with respect to the lighter ions and that therefore the time it takes for the heavier ions to pass through the aperture will have increased also, but that the spatial separation I has not changed. Finally, in considering only three species of ions, the picture remains reasonably clear. However, if the original packet d -c -d -b contains a large number of different species, then we will see a kink" emerge from the deflection plates which will initially be very much contained in space, i,e., a large number of kinks all corresponding to a particular mass will be superimposed upon each other. But with the passing of time, these kinks will separate from each other. Also the lighter ions at the left-hand side as viewed in FIG. 2, i.e., the d position, will overtake the heavier ions at the front of a kink, i.e., the b position. This effect will be especially pronounced along the path a m-11 d. a',-a etc. and e -e,e ,e -e',-e because of the mixing of the ions in the kink and the ions in the deflected beam. In other words, the heavy ions e -e and e,e will be continuously overtaken by lighter ions emerging from the plates after a field change has occurred. The whole interspace limited by a e -e e", and e -a,,-e' and a, will be temporarily filled with ions after a field change has occurred.

The ion-bunching method of the present invention has two outstanding featureslFirstly, because At is mass dependent the resolution will be independent of mass, and secondly because 1 is independent of mass the number of ions in each packet b-c-aJ-c is independent of mass and, therefore, the sensitivity of the mass spectrometer is independent of mass.

The foregoing description of FIG. 2 may be summarized in the following relationships:

If C C C .....C,, denote constants, then for a homogeneous ion beam of energy eU (we assume U =0 and U=U t,==C t =C t dependent on position of ion, t (b)=C t (d)=C 55 'yt==C For an ion beam of energy eU containing ions of different molecular weight m, where m=m m ...m,, then, t =C m t =C m t dependent on position of ion.

R=L Vo/2DU(B+S F0 1. I Spectral intensity is proportional to I'll or At/t- C wherein:

l effective length of electrical field between deflection plates (typically 3040 mm.)

D effective width of electrical field between deflection plates (typically 1-3 mm.)

l'= length of ion bunch L= Length of drift space 3 (typically 0.5-2 m.)

B Beam width (typically 1-5 mm.)

S= Width of slit 4 (typically 0-10 mm.)

U Predeflection accelerator voltage U Postdeflection accelerator voltage U =Total accelerator voltage (typically 2000-5 ,000 v.)

eU =Total ion energy (typically 2,000-5,000 ev.)

e =Charge of typical ion 1.60Xl0Coulomb) m Molecular weight of ion (typically l-800 a.m.u.)

a.m.u. =Atomic mass unit l.66 l0'kg).

V(tt) deflection plate voltage V0 =Magnitude of deflection plate voltage change 10-100 E V(t)D electrical field strength between deflection plates v (2eU/m) ion beam velocity t, l(m/2eU,)" =transit time in deflection plates of length t L(m/2eU) transit time of ions in drift space of length L.

t =time ion has spent within electrical field between deflection plates when electrical field change occurs.

t (b) time a particular ion has spent within electrical field between deflection plates when electrical field change occurs. In this case (b) denotes the first ion of a particular bunch to pass through slit 4.

t (d) =same as t (b), except this is last ion of a particular bunch to pass through slit 4.

At=t (b) t (d) ion bunch duration Turning now to FIGS. 3, 4 and 5, these show the effects of differing rise times in the step-function-controlled voltage applied to X- and Y-plates, and demonstrate that the rise time of the voltage change must be approximately equal to or less than half the transit time of the heaviest ions through the electrical field. In each case the time basis of the oscilloscope was 0.1 tsecJcm. and the transit time through the field of the Y- plates was approximately 450 nanoseconds. In FIG. 3 the rise time of the applied voltage was 17 nanoseconds and the sharpness of the oscilloscope trace and the high resolving power (800) are clear. In FIG. 4 the rise time was 300 nanoseconds and the indistinct nature of the pulses and the consequent loss of resolving power is beginning to be appreciable. In FIG. 5 the rise time was 500 nanoseconds and from this figure it is clear that the resolving power has decreased to a degree which is unacceptable.

FIGS. 6, 7 and 8 show the effect of pulse width, i.e., the time between step-functiomcontrolled voltage increase and decrease, when using only Y-plates and demonstrate the desirability of returning the step-function-controlled voltage to its original value only after the spectrum has taken place. In FIG. 6 is shown the oscilloscope display was obtained with a time base of l usec/cm. and a square pulse having a width of 0.5 usec; the oscilloscope trace due to the trailing edge of the pulse overlaps with that resulting from the leading edge of the pulse. In FIG. 7 the time base was the same as in FIG. 6 and the pulse width was 6 #sec. and it will be seen that the overlap between the two traces has been materially reduced in comparison with FIG. 6. The trace shown in FIG. 8 was produced with a time base of 5 used/cm. and a pulse width of 18 14sec.

and here the leading edge and the trailing edge traces are entirely separate.

FIGS. 9 and 10 show the effect on the oscilloscope trace (in both cases of l used/cm.) of pulse width when using X- and Y-plates, and demonstrate the desirability, when the stepfunction-controlled voltage has the form of a square pulse, of a pulse duration greater than the time of flight of the ions between the two sets of plates. ln FIG. 9 the pulse width was 1 sec. and this has given a satisfactory trace showing heavier ions on the right and lighter ions on the left. When this pulse width was reduced as shown in FIG. 10 to approximately 0.5 sec. the heavier and slower-moving ions have failed to reach the collector and are absent from the trace.

Although a straight flight path for the ion beam has been described in the foregoing, it is to be understood that there may also be used instead a helical flight path obtained by passing the ion beam into the space between two concentric tubes between which an appropriate electrical field is maintained.

I claim as my invention:

1. A time-of-flight mass spectrometer wherein a mass spectrum of ions is obtained from a material under investigation by accelerating a group of the ions to velocities corresponding to their mass-to-charge ratios, making the group cover the distance to a target to allow the ions with different mass-tocharge ratios to separate and measuring the current of ions that reach the target, and displaying the course in time of the measured ion current;

means for substantially continuously generating a beam of ions and accelerating such beam in the direction of a target area,

a pair of deflection plates being disposed so that the ion beam passes between the plates;

a source of voltage and means for varying the voltage as a step-function, said source being coupled to said deflection plates;

a target slit and current detector disposed in the target area traversed by the ion beam so that for each such step function a group of the ions passing through said electrical field pass through said target slit and current detector; and

means for recording the course in time of the current detector output signal.

2. The apparatus of claim 1 wherein said voltage source has a rise time equal or less than half the transit time of the heaviest ions through the field.

3. Time-of-flight mass spectrometer according to claim 2, characterized by a source of trigger pulses which are generated at instants corresponding to the step-function changes in the deflection voltage, said source of trigger pulses being connected to an input of the said means for recording the course in time of the ion current detector output signal.

4. Time-of-flight mass spectrometer according to claim 3, characterized in that the source of trigger pulses is capable of generating only trigger pulses at instants corresponding to step-function changes proceeding in one direction.

5. Time-of-flight mass spectrometer according to claim 4, characterized by another pair of deflection electrodes connected to a second source ofa step-function deflection voltage which, apart from possible phase shifts, can vary synchronously with the first source of voltage.

6. Time-of-flight mass spectrometer according to claim 5, characterized in that the two pairs of deflection electrodes are arranged to produce substantially perpendicular electrical fields.

7. Time-of-flight mass spectrometer according to claim 2, characterized in that the target is a slit in a diaphragm.

8. A method of repeatedly obtaining a mass spectrum of ions froma material under investigation by accelerating a group of the ions to velocities corresponding to their mass-tocharge ratios, making this group cover the distance to a target to allow ions with difierent mass-to-charge ratios to become separated owing to the different magnitudes oftheir velocities, measuring the current of ions that have reached the target by means of an ion current detector and recordin the course in the time of the measured ion current, improvement comprising; forming a series of the said ion groups by passing a beam of accelerated ions between two deflection electrodes and applying to these electrodes a deflection voltage which alternates between two levels chosen in such a way that the beam is deflected repeatedly towards, across and beyond the target and which describes, at least approximately, a stepfunction between the said levels in one or both directions of variation.

9. A method according to claim 8 characterized in that the said step-function as described by the deflection voltage has a rise time not longer than approximately half the transit time of the ions with the highest mass-to-charge ratio through the space between the deflection electrodes.

10. A method according to claim 9, characterized in that the ionbeam is deflected in substantially perpendicular directions by two pairs ofdeflection electrodes.

11. A method according to claim 8, characterized in that the changes in deflection voltage between the said levels are separated by time intervals chosen sufficiently long to prevent the periods of arrival at the ion current detector of successive groups of ions from overlapping at least during a part of interest of these periods.

12. A method according to claim 8, characterized in that the ion beam is passed between another pair of deflection electrodes to which a second deflection voltage is applied which varies synchronously with the first-mentioned deflection voltage, thereby preventing the first-mentioned deflection voltage in one direction of alternation between levels from giving rise to groups of ions that would reach the target.

13. A method according to claim 12, characterized in that the ions of the beam, after having passed between the deflection electrodes, are subjected to an electrostatic field so as to modify their velocities towards the target. 

1. A time-of-flight mass spectrometer wherein a mass spectrum of ions is obtained from a material under investigation by accelerating a group of the ions to velocities corresponding to their mass-to-charge ratios, making the group cover the distance to a target to allow the ions with different mass-to-charge ratios to separate and measuring the current of ions that reach the target, and displaying the course in time of the measured ion current; means for substantially continuously generating a beam of ions and accelerating such beam in the direction of a target area, a pair of deflection plates being disposed so that the ion beam passes between the plates; a source of voltage and means for varying the voltage as a stepfunction, said source being coupled to said deflection plates; a target slit and current detector disposed in the target area traversed by the ion beam so that for each such step function a group of the ions passing through said electrical field pass through said target slit and current detector; and means for recording the course in time of the current detector output signal.
 2. The apparatus of claim 1 wherein said voltage source has a rise time equal or less than half the transit time of the heaviest ions through the field.
 3. Time-of-flight mass spectrometer according to claim 2, characterized by a source of trigger pulses which are generated at instants corresponding to the step-function changes in the deflection voltage, said source of trigger pulses being connected to an input of the said means for recording the course in time of the ion current detector output signal.
 4. Time-of-flight mass spectrometer according to claim 3, characterized in that the source of trigger pulses is capable of generating only trigger pulses at instants corresponding to step-function changes proceeding in one direction.
 5. Time-of-flight mass spectrometer according to claim 4, characterized by another pair of deflection electrodes connected to a second source of a step-function deflection voltage which, apart from possible phase shifts, can vary synchronously with the first source of voltage.
 6. Time-of-flight mass spectrometer according to claim 5, characterized in that the two pairs of deflection electrodes are arranged to produce substantially perpendicular electrical fields.
 7. Time-of-flight mass spectrometer according to claim 2, characterized in that the target is a slit in a diaphragm.
 8. A method of repeatedly obtaining a mass spectrum of ions from a material under investigation by accelerating a group of the ions to velocities corresponding to their mass-to-charge ratios, making this group cover the distance to a target to allow ions with different mass-to-charge ratios to become separated owing to the different magnitudes of their velocities, measuring the current of ions that have reached the target by means of an ion current detector and recording the course in the time of the measured ion current, improvement comprising; forming a series of the said ion groups by passing a beam of accelerated ions between two deflection electrodes and applying to these electrodes a deflection voltage which alternates between two levels chosen in such a way that the beam is deflected repeatedly towards, across and beyond the target and which describes, at least approximately, a step-function between the said levels in one or both directions of variation.
 9. A method according to claim 8 characterized in that the said step-function as described by the deflection voltage has a rise time not longer than approximately half the transit time of the ions with the highest mass-to-charge ratio through the space between the deflection electrodes.
 10. A method according to claim 9, characterized in that the ion beam is deflected in substantially perpendicular directions by two pairs of deflection electrodes.
 11. A method according to claim 8, characterized in that the changes in deflection voltage between the said levels are separated by time intervals chosen sufficiently long to prevent the periods of arrival at the ion current detector of successive groups of ions from overlapping at least during a part of interest of these periods.
 12. A method according to claim 8, characterized in that the ion beam is passed between another pair of deflection electrodes to which a second deflection voltage is applied which varies synchronously with the first-mentioned deflection voltage, thereby preventing the first-mentioned deflection voltage in one direction of alternation between levels from giving rise to groups of ions that would reach the target.
 13. A method according to claim 12, characterized in that the ions of the beam, after having passed between the deflection electrodes, are subjected to an electrostatic field so as to modify their velocities towards the target. 